The injection of steam to recover oil from heavy oil formations is an accepted method in the industry. Past experiments and field performance have shown the improved displacement efficiency of heavy oils by reduction in viscosity of the oil by a heated displacing phase. Displacement of oil increases with increasing temperature.
Steam is considerably lighter than the oil and water present in the formation and thus, because of gravity segregation, it tends to rise to the top of the formation when vertical communication exists. Consequently, the injected steam channels through the top of the formation to the producing well overriding a major portion of the formation and contacting only a small fraction of the formation oil. Once steam override has begun, continued injection of steam into the formation will accomplish very little additional oil recovery. This behavior results in an inefficient oil recovery and low vertical sweep efficiency.
U.S. Pat. No. 4,175,618 discloses a high vertical and horizontal conformance thermal oil recovery process wherein steam is injected into a formation until channeling has begun, steam injection is terminated and an aqueous fluid is injected into the formation, which has a viscosity essentially the same as water and contains an emulsifying agent capable of forming a viscous emulsion in the portion's of the formation when override, underride or channeling has occured. After a small slug of emulsifying fluid has been injected, steam injection is resumed and as a consequence of the emulsion plug, moves into portions of the formation from which oil would not be recovered if the emulsion had not been injected. Multiple cycles of steam and emulsion injection may be applied to a formation to more completely sweep the full volume of the pattern.
The following U.S. patents describe processes for forming viscous emulsions on the surface of the earth and injecting them into formations for the purpose of decreasing the permeability of zones in the formations which are more permeable than other portions of the formation. U.S. Pat. No. 3,149,669; U.S. Pat. No. Re. 27,198 (original U.S. Pat. No. 3,443,636); U.S. Pat. Nos. 3,502,146 (1970); and 3,866,680 (1975). U.S. Pat. Nos. 3,946,812; 3,888,308; 3,508,612; 3,500,923; 3,827,497; 3,811,504 and 3,890,239 relate to the use of sulfated ethoxylated surfactants in oil displacing fluids. U.S. Pat. Nos. 3,792,731; 3,799,264; 3,811,504 and 3,811,505 describe oil recovery processes and fluids employing non-ionic surfactants. U.S. Pat. Nos. 3,827,497 and 3,890,239 disclose the use of a mixture of organic sulfonate and a sulfated or sulfonated, ethoxylated alcohol.
Viscous polymer fluids are not effective for treating steam-swept zones in connection with thermal oil recovery methods because such polymers are not suitable at the temperatures which are encountered in the steam-swept zone. The viscosity developed by polymers in fluids prepared on the surface of the earth is lost quickly in the subterranean formation after it had been exposed to the elevated temperatures of the steam swept zone. Viscous emulsions are usually unsuccessful in treating permeability problems created in viscous oil formations by steam flooding because the emulsions are difficult to inject into the formation since their viscosity is highest at the time of injection, and so the effectiveness is mostly limited to the portions of the formation very near the wellbore.
The present invention provides a method for increasing the sweep efficiency of a steam drive oil recovery method utilizing a thermally stable diverting fluid.